1. Field of the Invention
This invention relates to a method of and an apparatus for measuring the refractive power of a contact lens, particularly, a soft contact lens.
2. Description of the Prior Art
A lens meter for measuring the refractive power of spectacles lenses has heretofore been used to measure the refractive power of a contact lens, but this method is limited to the case of the so-called hard contact lens which is formed of hard material such as PMMA (polymethyl methacrylate) and which is small in amount of deformation. On the other hand, in the case of a flexible hydrophilic soft contact lens formed typically of HEMA (2-hydroxyethyl methacrylate), it is difficult to measure the refractive power thereof by the use of a lens meter as in the case of a hard contact lens, due to the effect of drying and instability of shape attributable to the flexibility when the soft contact lens is taken out of the preservation liquid such as physiological saline solution. Even if the measurement could be made, there is a problem of accuracy because the shape of the soft contact lens is unstable. For these reasons, the immersion method using physiological saline solution or the like has been proposed for the measurement of a hydrophilic soft contact lens. In this method, the amount of deformation of the lens is much smaller than it is in the air because the specific gravity of the liquid is approximately that of the soft contact lens but, nevertheless the refractive indices of the lens and the liquid must be previously known to find a rough value. If these are unknown, comparative measurement of the degree of the refractive power of the lens can be made, whereas the value of the refractive power itself cannot be obtained. Other various measuring methods have been considered, but various problems are still left to be solved such as the fact that measurement cannot be made of a lens whose refractive index is unknown, that the accurancy of measurement is low because the difference in refractive index between the liquid and the lens is small, and that handling is cumbersome.